Discover Top Posts Tagged with #hexanchidae

#2778 - Fam. Hexanchidae - Cow Shark Tooth

From the Weka Pass limestones, deposited in the mid-Oligocene Duntroonian Stage (27-25Mya).

The Hexanchid sharks are distinguished from other shark families by the six, and sometimes seven, gillslits, compared to the five of other sharks. That doesn't help much with the extinct ones, since complete fossils are extremely rare and most of the ones we do have are loose teeth - compressed and comb-like teeth from the lower jaw and smaller and pointed from the upper jaw.

Cow Shark diet usually consists of large fish, including other sharks. Anatomically they're conservative and 'primitive', and may be the oldest remaining shark family - maybe even survivors of the Permian Great Dying.

University of Otago Geology Museum, Dunedin, Aotearoa New Zealand.

#Hexanchidae #cow shark #sixgill shark #University of Otago Geology Museum #Dunedin #Dunedin NZ #new zealand fossil #fossil #fossil shark

GOD I love cow sharks I researched them for like three days a few years ago and they're SO cool i'm gonna research them again so I can talk more about them and annoy everyone around me

#autistic evil #my evil knows no bounds #hexanchidae #cow shark #sharks #elasmobranch

Notorhynchus cepedianus

#notorhynchus cepedianus #broadnose sevengill shark #hexanchidae #sharks #Oregon Coast Aquarium

A really fascinating correlation I've just put together is that hexanchid teeth have been discovered from Antarctica dating back to the Eocene epoch, 56-33MYA. Cetaceans likely evolved into their own distinct clade 50MYA from the Indian continent. Since Hexanchus griseus has never been observed eating live prey, only dead animals (including most famously dead whales, from Blue Planet 2), I wonder if the evolution of hexanchiformes occurred in response to the development of cetaceans. Their teeth are well suited to the task of separating large chunks of flesh from bodies.

Evidence of a shark that feeds exclusively on ichthys is Carcharias taurus, who has thin, pointy teeth capable of trapping prey in is mouth. Compared against a marine mammal hunter, like Carcharodon carcharias, it has spade-like teeth that are optimal for breaking the epidermis and holding onto struggling prey. The teeth of hexanchidae sharks are optimal for grasping (upper teeth) and then sawing (lower teeth). Considering the pelagic nature of cetaceans, they are likely to perish above deep waters, where adult Hexanchus griseus dwells.

Stomach contents of juvenile H. griseus consist of benthic fish and invertebrates, which also suggest ontogenetic diet changes. This is fascinating, and is absolutely going to be added to my list of H. griseus research objectives.

Hypothesis: Hexanchiformes evolved from sharks who became increasingly reliant on cetacean carrion for sustenance.

Null: No correlation exists between the evolution of hexanchiformes and cetaceans.

Any critiques?

#hexanchus griseus #hexanchidae #hexanchiformes #sharks #bluntnose sixgill shark #marine biology #paleontology #cetaceans #whales

The crabs in this video are hilarious

#chondrichthyes #selachii #hexanchiformes #hexanchidae #hexanchus griseus #h. griseus #bluntnose sixgill shark #cowshark #sharks #fish #cancer productus #red rock crab #invertebrate #crustacean

The Bluntnose Sixgill Shark (Hexanchus griseus) is a widely distributed demersal species whose population biology is poorly understood. Although H. griseus is normally found in deep continental slope waters, individuals from a population in the Strait of Georgia, British Columbia, make unexpected diurnal movements onto a shallow reef (Flora Islets) between June and August. This shallow water activity allowed in situ length measurements to be made on 35 free-swimming Bluntnose Sixgill Sharks using stereo videography. The measured sharks were all large juveniles and sub-adults, although smaller juveniles and pregnant females are known to occur in deeper adjacent waters. The restricted size distribution at Flora Islets may arise because small juveniles avoid contact with larger conspecifics and mating takes place offshore. All measured sharks were individually identified by unique scar patterns. In 13 of 35 sharks these scars were consistent with injuries expected from hooking and entanglement by commercial fishing gear.

This paper is very similar to the last, and is even authored by the same researchers. Using a stereo-camera construction at the Flora Islet reef wall, they were able to measure the total length of 35 free-swimming Hexanchus griseus and also examine scars on the sharks.

The length-frequency relationship observed resembled a bell curve, with the most frequently occurring total length being 180cm-299cm. That makes most of these shark subadults and juveniles. 13 of the 35 H. griseus bared scars similar to those caused by commercial fishing gear.

#chondrichthyes #selachii #hexanchiformes #hexanchidae #hexanchus griseus #h. griseus #bluntnose sixgill shark #cowshark #bc #british columbia #strait of georgia #flora islet

The bluntnose sixgill shark (Hexanchus griseus) is a large, deepwater, cosmopolitan species whose range includes the Strait of Georgia, British Columbia. Although this species is probably the top predator on most of the world’s continental shelves, there is little information on its population biology or behaviour. At a unique site off Flora Islets, in the Strait of Georgia, a number of H. griseus individuals show unexpected diurnal activity on a shallow reef. Regular movements of these sharks along the base of the reef wall allowed us to use an automated, time-lapse video system to make the first extensive observations of their behaviour between June 2001 and July 2002. Sixgill shark frequency (no. of sharks observed/h) increased in June and reached a peak between mid-June and mid-July. Sixgill sharks were not observed between October and May. Sixgill shark frequency was highest in the afternoon and the sharks tended to move as individuals rather than in groups. The observations suggest that this shallow-water activity is not related to either reproduction or feeding and its purpose remains unclear.

We return to the Pacific Northwest with this 2003 paper from renowned diver and sixgill enthusiast Zielinski!

This study took place on a shallow reef of the Flora Islet from June 2001 to July 2002. A time-lapse camera was fixed to the reef wall 40m down and recorded an image every 9s. By doing so, they were able to get 240h of data on a 4h footage tape (yes, tape! Like a VCR!). The storage medium was fix at 20m on the reef wall. The recording period was 12h each day from 06:00 to 18:00. The reason for this is visibility. Outside these hours, it would not be possible to distinguish shapes on the film. That being said, this data does not account for nighttime activity. Only daytime activity.

So, what did they find? The most Hexanchus griseus activity on the reef (measured in inbound sharks per hour) occurred in the afternoon from mid-June to mid-July. No daytime activity was seen between October and May. None of the recorded H. griseus were mature, based on estimated TL from footage. Thus, this activity was not based on reproduction. No feeding activity was observed, either, despite the presence of known food items, such at Hydrolagus colliei, Squalus suckleyi, and Phoca vitulina. There's some ambiguity on the conclusion here. Just because this one camera angle didn't capture feeding doesn't mean feeding didn't occur. The likely reason for this pattern of diurnal activity, thus, is temperature regulation. The water temperature between 75 and 200m in the Strait of Georgia is about 8-9 degrees Celsius year round, whereas surface temps can get as cold as 6-7 in Winter and as high as 16-18 in the Summer.

This is really valuable information. If this can be confirmed, it could inform conservation efforts for Hexanchus griseus in the face of ocean temperature increase from climate change and global warming. The reef itself has seen a decline in biodiversity and fauna size over the last 150 years. This is altogether interesting, considering that Hexanchus griseus were thought to stay quite deep during the day, even in Summer.